I think it makes sense to get a gut feeling of the founding team (Can they sell? Do they have a passion for the space? Do they seem like they are in it for the long haul? Do they seem willing to pivot?) and it makes sense to hear/believe the narrative of the market size ( http://is.gd/ceLQU ), etc. Every investor is going to have the secret trait/quality in a founder/market that they think is critical.
But trying to guess what founders and markets are going to do over a period of 7+ years shouldn't be about dozens of hours of diligence.